In the past, polypropylene closures have been made with a compression-molded liner on the inside of the closure top. It is desirable to have a liner that is removable for some applications. However such liners were not removable and removal efforts resulted in tearing and destruction of the liner material. Apparently the polypropylene of the cap and the material in the liner fused and formed a strong bond from the heat and pressure of the compression molding operation.
It is highly desirable to have a good efficient liner that can be easily removed for some applications.